Thin film solar cells show high energy conversion efficiency and have little deterioration in efficiency due to light irradiation or the like when CuInSe.sub.2 (CIS), i.e. a compound semiconductor thin film (chalcopyrite based semiconductor thin film) containing group Ib elements, group IIb elements and group VIb elements, or Cu(In,Ga)Se.sub.2 (CIGS), in which Ga is dissolved in CuInSe.sub.2, is used as a light absorbing layer in the solar cells. The solar cells containing the above-mentioned compound semiconductor are explained in detail by Bloss et. al. (in "Progress in Photovoltaic". 3 (1955), p. 3). These thin film solar cells have better conversion efficiency when they had a substrate structure such as a glass/back electrode (Mo)/CIS-based thin film or CIGS-based thin film/CdS/transparent conductive film (for example, ZnO/ITO or ZnO/ZnO:Al) structure.
It is well known that the condition of the surface layer of a CIS film or a CIGS film highly influences the conversion efficiency of solar cells. For instance, in CIGS-based solar cells, conversion efficiency is highly dependent on the conditions of a pn junction formed between a p-type CIGS film and a so-called n-type window layer semiconductor, and particularly, is dependent on the conditions of the surface layer (which becomes a pn junction interface) of a CIGS film. Thus, the surface layer of these semiconductor thin films has been focused on in research. At the 11th E.C. Photovoltaic Solar Energy Conference held Oct. 12-16, 1992 in Montreux, Switzerland, H. W. Schock et. al. reported that a CuIn.sub.3 Se.sub.5 compound layer exists on the surface of a CIS film, which is formed by a deposition method, in the paper titled, "HIGH EFFICIENCY CHALCOPYRITE BASED THIN FILM SOLAR CELLS RESULTS OF THE EUROCIS-COLLABORATION." It was also suggested that this CuIn.sub.3 Se.sub.5 shows n-type conduction and constitutes a pn junction with the p-type CIS. It can be considered that this surface layer lessens the defects of a pn junction interface and contributes to the high conversion efficiency of CIS-based solar cells.